Distributed Temperature Sensing (DTS) is widely used across many industries and applications where it is beneficial to collect a large number of temperature points along a structure or process. An optical fiber is used as a sensor and placed in the area where it is desirable to measure the temperature. A short light pulse at a center wavelength is transmitted down the fiber where it interacts with the structure of the fiber and some of the energy is shifted to different wavelengths and scattered back along the fiber through e.g. Raman scattering. Some of the energy is shifted to a higher wavelength called the Stokes wavelength, and some is shifted to a lower wavelength called the anti-Stokes wavelength. The temperature can be calculated as a function of the ratio of the Stokes and anti-Stokes wavelength. The Stokes and anti-stokes signals must be corrected for the wavelength dependent losses along the fiber, and this is often done using the assumption that the difference in optical attenuation between the Stokes and anti-Stokes component is constant over the distance of the fiber with some compensation for thermal effects. The position of the temperature along the fiber is determined by measuring the time of flight between the transmitted and reflected light, and given that the speed of light is known, the location can be calculated. The way to determine the position for Raman based Optical Time Domain Reflectometry (OTDR) DTS technology is similar to what is used in commercially available Optical Time Domain Reflectometry (OTDR) units based on Rayleigh scattering.
The returning backscattered light is converted to an analogue electrical signal using a photo-diode and an electrical amplifier. The analogue signal is digitized using an Analogue to Digital Converter (ADC). The sampling frequency of the ADC determines the sampling resolution of a given system. The laser pulse width and the ADC sampling frequency determine the spatial resolution of a system, i.e. the distance it takes for the system to fully respond to a step change in temperature. The spatial resolution for OTDR based systems used both in the sensing and telecommunications industry is normally on the order of a few meters for high performance system.
Many systems have in the past used the transmitted laser pulse as a trigger mechanism for the start of the data collection by the ADC card. These systems use an additional photodiode to time the laser pulse and convert this into an electrical signal, which in turn is used to start the data collection of the ADC card. This approach adds a signal jitter in the order of +/−1 sampling point, which further drives the demand for a high clock frequency ADC card.
High sampling frequency ADC cards are more expensive than lower sampling frequency ADC cards. The heat generated by the ADC cards is in general proportional to the clock frequency and higher frequency systems normally generate more heat than lower frequency systems. High performance systems tend to be expensive and generate a lot of heat while many applications demand cost effective low power consumption solutions for extended environmental range performance. The lack of cost-effective systems with adequate performance have over many years limited the use of DTS systems in many applications.
Another drawback of the existing systems is the coarse sampling resolution and the impact it has on double ended correction, both on the noise and the spatial resolution. Double ended correction is used to mitigate e.g. hydrogen induced darkening in fibers. An optical fiber is deployed in a loop configuration with both fiber ends tied to the DTS instrument, light is injected from one fiber end and a temperature trace is collected. Light is then injected from the second fiber end and a temperature trace is then collected. Using this data, a differential attenuation factor between the Stokes and anti-Stokes wavelengths can be calculated over the length of the fiber. This differential attenuation factor allows the user to correct for changes and/or non-uniformities of the optical attenuation along the fiber. The two temperature traces must be well aligned to get an accurate and low noise temperature trace where small amounts of misalignment will cause a significant increase in noise. This misalignment between the two temperature traces will also cause degradation in spatial resolution.